Methods for production of zinc oxide electrodes for alkaline batteries

ABSTRACT

A zinc electrode for use in alkaline batteries comprises a mixture of 0.425 to 1.55 volume parts of zinc oxide with a volume part of a metallic oxide chosen from the group consisting of: calcium oxide, barium oxide, and mixtures thereof, together with hydroxy-ethyl cellulose, an oxide dispersant chosen from the group consisting of: soap derivatives, anionic polyelectrolytes, anionic surfactants, and mixtures thereof, and a binder. The electrode is prepared by mixing zinc oxide with the chosen metallic oxide in an aqueous medium such as water or potassium hydroxide, stirring overnight, filtering and drying the mixture, optionally adding a further small amount of zinc oxide, optionally adding other metallic oxides, and adding hydroxy-ethyl cellulose, an oxide dispersant, and a binder. The aqueous paste os slurry thus formed is placed on a conductive substrate, drawn through a sizing gap, cut and dried, to form low cost pasted zinc oxide electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/276,344 filed Mar. 15, 2001 for Jeffrey Phillips andtitled “Methods for Production of Zinc Oxide Electrodes for AlkalineBatteries,” through International Application No. PCT/CA02/00352 filedMar. 15, 2002.

FIELD OF THE INVENTION

This invention relates to zinc oxide electrodes for use in alkalinebatteries, and provides methods for production of such zinc oxideelectrodes as well as the formulations thereof. Particularly, thepresent invention provides for the low cost production of pasted zincnegative electrodes.

BACKGROUND OF THE INVENTION

A conundrum develops with respect to the use of zinc electrodes in highrate, high performance batteries. On the one hand, the use of zincelectrodes provides low cost negative electrodes. On the other hand,when such electrodes are pasted, as is usually the case, they are verydifficult and very expensive to make, especially when steps are taken toavoid shape change as will inevitably occur while the zinc electrode iscycled during the life of the rechargeable battery in which it isplaced.

The present inventor has unexpectedly discovered that mass production ofenvironmentally benign zinc electrodes can be provided for, by adaptingcertain aqueous pasting techniques, and by the provision of a stablezinc-based slurry or paste which particularly has an additional oxide ofcalcium, magnesium, or barium included therein for purposes ofstability.

It is well known that rechargeable batteries having zinc electrodes havethe advantage of using a low cost negative electrode. Of course, theyalso provide high voltage, and light weight.

On the other hand, the zinc electrode in alkaline batteries will exhibitsome solubility in most alkaline electrolytes, resulting in the loss ofintegrity of the zinc electrode as the battery is cycled. Thisphenomenon of zinc electrode disfigurement is well known, and isreferred to in the industry as shape change.

Many approaches have been taken to control shape change. The mostsuccessful attempts have limited the solubility of the zinc electrode,so as to thereby reduce its ability to migrate.

For example, EISENBERG U.S. Pat. No. 4,224,391, issued Sep. 23, 1980,and U.S. Pat. No. 5,215,836 issued Jun. 1, 1993, each provideselectrolyte formulations that employ mixtures of potassium hydroxide andboric acid, phosphoric acid, or arsenic acid. The purpose, in allevents, is to limit the solubility of the zinc electrode.

Another approach is that taken by ADLER et al in U.S. Pat. No.5,302,475, issued Apr. 12, 1994. In that patent, the inventors teach anelectrolyte which contains potassium hydroxide and a combination ofpotassium fluoride and potassium carbonate salts. The zinc electrode isreported to exhibit low shape change.

Yet another approach which is known to the inventor, is the addition ofmaterial such as calcium oxide to the electrode itself. This approachwill modify the chemical composition of the discharge product toeffectively lower the concentration of the zinc ion in the solution.However, in order to do so a method must be devised whereby a calciumzincate water based paste or slurry is manufactured, for pasting intothe conductive substrate of a zinc electrode.

Moreover, the method by which the zinc electrode is manufactured mayhave significant effect upon cycle performance of the battery. A numberof preparation processes have been used to further optimise theperformance of zinc electrodes. They include powder pressing as taughtby HEIN et al U.S. Pat. No. 3,645,793, issued Feb. 29, 1972. In thatpatent, the inventors teach pressing metallic powder into highly porousmetallic battery electrodes structures, and the powders are firstcleaned by the action of mild acids prior to and during the pressingoperation.

Another approach is that taken by CHARKEY et al in U.S. Pat. No.5,863,676, issued Jan. 26, 1999. Here, a non-aqueous sheet formation isemployed, where a calcium zincate constituent is formed external of thebattery, and then placed in the battery in layers.

However, any process which employs a pasting operation is difficult tocontrol as far as managing the “set” time during pasting is concerned.This is reported in a paper entitled Pasted-Rolled Zinc ElectrodesContaining Calcium Hydroxide for Use in Zn/NiOOH Cells by Gagnon et al,published in the Journal of the Electrochemical Society, September 1987,at pages 2091 to 2096. It is reported at page 2092 that the electrodefabrication was carried out in such a manner that the same amount ofpaste is added to both sides of a grid, by sandwiching the paste betweenbacking papers and passing it through rollers to make a thin film.Thereafter, the current collector was placed between 2 films, and thecomposite was pressed to make a zinc electrode. This provided a longerset-time in which to paste the electrodes, and adhered the paste to thecurrent collector grid in satisfactory manner. The thickness of theelectrode, and therefore the zinc loading density, was controlled by thegap between the rollers.

Regrettably, none of the above mentioned processes can be readilytransferred to mass production techniques, for high rate production ofzinc electrodes for use in high performance batteries. Therefore, it hasnot been possible to provide a commercial product at a reasonable price,even though the chemical problems of the shape change of the zincelectrode have been effectively resolved.

The present invention provides a method for mass production of anyenvironmentally benign zinc electrode, where the electrode willdemonstrate excellent mechanical characteristics, good rate capability,and high cycle life, without significant shape change.

Indeed, the methods of the present invention may be readily and easilyadaptable to all rechargeable zinc battery production, so that in timelow cost zinc-based batteries may be manufactured in substantialquantities, and thereby replace toxic cadmium-based batteries. Suchcadmium-based batteries enjoy a strong market position, even now, due totheir relatively low cost, notwithstanding their toxicity andnotwithstanding memory effect, and the like.

It is very difficult to uniformly coat an aqueous paste of zinc oxideonto a metallic current collector. When calcium oxide or calciumhydroxide is added to the mix, the problem of uniformly coating anaqueous paste zinc oxide onto a metallic current collector is even moredifficult.

These problems come as a consequence of the rapid solidification whichmay prevent further work with the mix, or the final pasted plate may bestructurally unsound in such a manner that further handling isimpossible.

These problems, of course, have lead to non-standard manufacturingtechniques in the manufacturer of zinc electrodes, thereby adding to thecosts of the manufacture of the zinc electrodes. However, if asuccessful adoption of aqueous based manufacturing techniques such asthose which are applied to cadmium electrodes could be developed forzinc-based electrodes, then a reliance on toxic cadmium-based batterieswould be overcome.

The present invention provides such a solution. In keeping with thepresent invention, it is possible to manufacture a continuous coatedelectrode without setting problems, and to provide a mechanically soundelectrode. Briefly, the present invention provides for a calcium zincateor other precursor constituent for an aqueous solution, based primarilyon mixtures of zinc oxide with calcium oxide, magnesium oxide, or bariumoxide, together with further optional metal oxides, and appropriatepasting constituents as a cellulose compound, a dispersant, and abinder.

SUMMARY OF THE INVENTION

To that end, the present invention provides both a method for productionof zinc oxide electrodes, and the zinc oxide electrodes themselves, foruse in alkaline batteries. The method comprises the following steps:

(a) Preparing an aqueous suspension of a metallic oxide chosen from thegroup consisting of: calcium oxide, magnesium oxide, barium oxide, andmixtures thereof, by placing 0.1 to 2.0 volume parts of the chosenmetallic oxide in 20 volume parts of an aqueous medium chosen from thegroup consisting of: water, and a 20% potassium hydroxide solution. Thechosen metallic oxide in the aqueous medium is stirred vigorously so asto form a suspension.

(b) Adding zinc oxide to the suspension, in an amount of 0.4 to 1.5times the volume of the chosen metallic oxide in the suspension.

(c) Stirring the suspension for a period of 1 to 12 hours.

(d) Filtering the stirred mixture.

(e) Drying the filter mixture at a temperature of 45° C. to 75° C., toform a powder mixture.

(f) Adding an additional 0.025 to 0.05 volume parts of zinc oxide to thepowdered mixture.

(g) Optionally, adding a further 0.01 to 0.04 volume parts of a furthermetallic oxide chosen from the group consisting of: bismuth oxide,indium oxide, tin oxide, and mixtures thereof, to the powdered mixture.

(h) Preparing an aqueous paste of the powdered mixture, by addinghydroxy-ethyl cellulose, an oxide dispersant chosen from the groupconsisting of: soap derivatives, anionic polyelectrolytes, anionicsurfactants, and mixtures thereof, together with a liquid binder, to thepowdered mixture.

(i) Coating or pasting the aqueous paste thus formed onto a conductivesubstrate.

(j) Drawing the coated or pasted conductive substrate through a gaphaving a predetermined dimension so as to fix the thickness of theelectrode being manufactured.

(k) Drying and cutting the electrode to size.

In keeping with provisions of the present invention, the aqueous mediummay be water, or it may be potassium hydroxide solution, as noted. Whenthe aqueous medium is water, then step (c) is carried out at roomtemperature. However, when the aqueous medium is potassium hydroxidesolution, then step (c) is carried out at 50° C. to 75° C.

Typically, the liquid binder provided in the formulation for zincelectrodes in keeping with the present invention, is dispersed PTFE.

The present invention also provides for a zinc electrode for use inalkaline batteries, which zinc electrode is characterised by a mixtureof 0.425 to 1.55 volume parts of zinc oxide with 1 volume part of ametallic oxide chosen from the group consisting of: calcium oxide,magnesium oxide, barium oxide, and mixtures thereof, together withhydroxy-ethyl cellulose, an oxide dispersant chosen from the groupconsisting of: soap derivatives, anionic polyelectrolytes, anionicsurfactants, and mixtures thereof, and also together with a binder.

Again, typically, the binder is dispersed PTFE.

The zinc electrode of the present invention may further comprise 0.01 to0.04 volume parts of a further metallic oxide chosen from the groupconsisting of: bismuth oxide, indium oxide, tin oxide, and mixturesthereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novel features which are believed to be characteristic of thepresent invention, as to its structure, organization, use and method ofoperation, together with further objectives and advantages thereof, willbe better understood from the following discussion.

Several examples now follow showing specific utilization of particularselected constituent components of zinc electrodes, and the method ofmaking the same, in keeping with the present invention.

EXAMPLE 1

100 grams of calcium oxide was added to about 2 liters of water, so asto form a suspension. The suspension was then stirred vigorously.

Thereafter, approximately 250 grams of zinc oxide—about 2.5 times theweight of calcium oxide—was then added to the suspension. It will beseen that approximately 0.32 volume part of calcium oxide andapproximately 0.47 volume parts of zinc oxide were employed.

The suspension was then continuously stirred overnight, at roomtemperature.

Thereafter, the solid mixture from the suspension was filtered, anddried at 60° C.

Then, another approximately 20 grams of zinc oxide was added to thepowdered mixture of zinc oxide and calcium oxide—being anotherapproximately 0.04 volume parts, for a total of about 0.51 volume partsof zinc oxide.

Also, approximately 28 grams of bismuth oxide—about 0.01 volumeparts—was added to the powdered mixture.

Then, a smooth paste was prepared by adding hydroxy-ethyl cellulose, anda dispersant. Various dispersants were used, including soap derivatives,anionic polyelectrolytes, anionic surfactants, and other surface activeagents.

Finally, a liquid binder—in this case, dispersed PTFE—was added.

EXAMPLE 2

In a second test, calcium oxide and zinc oxide were mixed in the sameproportion as noted above, but they were added to a 20% potassiumhydroxide solution. This mixture was then heated to 60° C. and stirredovernight to promote the formation of calcium zincate.

Thereafter, the mix was filtered and dried at 60° C.; an additionalsmall amount of zinc oxide, and an additional amount of bismuth oxide,were also added to the mix, in the same manner as set forth in Example1.

The mixes from both of the examples noted above were then formed into asmooth paste or slurry.

Thereafter, the paste or slurry was coated onto a suitable substratesuch as perforated metal, or pasted into a suitable substrate, in knownmanner.

However, typically, the mix was prepared as a slurry coat, and thesubstrate metal was drawn through the paste or slurry, and through aprecision gap of known dimension, so as to control the thickness of theelectrode being manufactured.

Thereafter, the pasted electrode were dried, cut to size, provided withan electrical contact, and assembled into electrochemical cells. Forpurposes of the tests conducted in keeping with the present teachings,the cells had pasted nickel positive electrodes, and were filled with anelectrolyte having 20% potassium hydroxide and 2% lithium hydroxide.

The test cells were cycled and tested for rate capability and cycleperformance, and were found to perform well, without significant shapechange of the negative electrode.

Thus, the present invention provides for the use of an aqueoussuspension of zinc oxide together with calcium oxide, magnesium oxide,or barium oxide, or mixtures thereof. The aqueous medium may be water orpotassium hydroxide.

Additional metal oxides such as bismuth oxide, indium oxide, tin oxide,and mixtures thereof, may also be provided in the aqueous pastepreparation, whereby further dimensional stability of the electrode isassured during repeated cycling operations.

As noted, typical cellulose compounds, dispersants, and binders, of thesort well known in the art, are employed with the inventive mixture, inthe preparation of a paste or slurry. However, because the paste orslurry is aqueous, and the pasting or coating operation can be carriedout at room temperature, then the set time for the paste is less of aproblem then hitherto experienced.

Methods of preparation of zinc electrodes for alkaline batteries, andthe zinc electrodes per se, have been described above. It will berecognised by those skilled in the art that other modification can bemade without departing from the spirit and scope of the appended claims.

For example, discussion of approximate volume part measurements isintended to suggest that there is no specific preciseness or exactitude,and that in any event the weight of equal volume parts of differingmetallic oxides will vary as a consequence of the differing atomicweights of the metals being employed.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not to theexclusion of any other integer or step or group of integers or steps.

1. A method for production of zinc oxide electrodes for use in alkalinebatteries characterized by the steps of: a) preparing an aqueoussuspension of a metallic oxide chosen from the group consisting of:calcium oxide by placing the chosen metallic oxide in an aqueous mediumto form the aqueous suspension; b) adding zinc oxide to the aqueousmedium of the chosen metallic oxide; c) stirring the aqueous suspensionto promote formation of calcium zincate; d) drying the stirred mixture;e) preparing an aqueous paste or slurry comprising the dried mixturefrom d), an oxide dispersant, and a liquid binder and f) coating orpasting the aqueous paste or slurry onto a conductive substrate.
 2. Themethod of claim 1, wherein when said aqueous medium is water, step (c)is carried out at room temperature; and when said aqueous medium ispotassium hydroxide solution, step (c) is carried at 50° C. to 75° C. 3.The method of claim 1, wherein said liquid binder is dispersed PTFE. 4.The method of claim 1, wherein the aqueous medium employed in a)comprises water or a potassium hydroxide solution.
 5. The method ofclaim 1, wherein a) comprises placing 0.1 to 2.0 volume parts of thechosen metallic oxide in 20 volume parts of the aqueous medium.
 6. Themethod of claim 1, wherein b) comprises adding zinc oxide to the aqueoussuspension in an amount of 0.4 to 1.5 times the volume of the chosenmetallic oxide in the suspension.
 7. The method of claim 1, wherein c)comprises stirring the aqueous suspension for a period of 1 to 12 hours.8. The method of claim 1, wherein d) comprises: filtering the stirredmixture; drying the filtered mixture at a temperature of 45° C. to 75°C. to form a powdered mixture.
 9. The method of claim 1, furthercomprising adding an additional 0.025 to 0.05 volume parts of zinc oxideto the dried mixture.
 10. The method of claim 1, further comprisingadding a further metallic oxide chosen from the group consisting of:bismuth oxide, indium oxide, tin oxide, and mixtures thereof, to thedried mixture.
 11. The method of claim 1, wherein the oxide dispersantis selected from the group consisting of: anionic polyelectrolytes,anionic surfactants, and mixtures thereof.
 12. The method of claim 1,wherein the aqueous paste or slurry further comprises a cellulosecompound.
 13. The method of claim 12, wherein the cellulose compound ishydroxyl-ethyl cellulose.
 14. The method of claim 1, further comprising:g) drawing the coated or pasted conductive substrate through a gaphaving a predetermined dimension so as to fix the thickness of theelectrode being manufactured; and h) drying and cutting the electrode tosize.